Generally, it has been known to provide optical observing devices and optical photographing devices such as endoscopes, cameras and television cameras with detecting means to determine when the device is in focus. Specifically, these detecting means have been provided to determine whether the image-forming position of the image-forming or photographing optical system of the device is in a focused state in which an image is clearly formed on an image-forming surface, such as a film surface for the object being observed.
In the past, it has been difficult to detect at a high precision with a simple structure whether a focused state is obtained in which the optical image of an object is clearly formed on the image-forming surface for photographing or observation.
For example, in a conventional focusing detecting device, should the object to be photographed be dark or use made of a dark photographing optical system, the detecting output of a light-receiving element receiving the image becomes so small that detection of the focused state is substantially impossible. To overcome such problems, it has been known to project a beam of a fixed shape toward the object to be photographed in a manner as described in the laid open Japanese Patent Application No. 19810/1974. In this case, use is made of an optical system in which the projecting means is independent of the photographing lens system. However, such a construction can hardly be used in an endoscope or the like in which photographing must be made in a single light path.
It has also been known to use split prisms, such as described in the laid open Japanese Patent Application No. 128923/1981. However, in case a precision above a fixed level is required, then a light-receiving element will be required. Further, the circuit system for detecting whether focusing is achieved or not by comparing output signals from the light-receiving elements will be complicated. Still further, for a small number of products, the cost will be high.
It has also been known to construct a focusing detecting device wherein a light source is flickered in order to project lights onto an object to be photographed. In this case, the output signals in the extinguished sections are deducted from the output signals in the lighted sections in order to reduce the influence of external lights other than of the light source. In this way, focusing may be detected even in case the object to be photographed is dark or if a dark photographing optical system is used. Such a system which employs a flickering light source is described in laid open Japanese Application No. 125713/1981. However, in this construction, this system requires two positions conjugate with a predetermined image-forming surface (e.g., a film surface) on both sides of the image-forming surface. Thus, the light projected onto an object by a light-projecting means arranged in one position may be received by the light-receiving means arranged in the other position. As a result, there have been problems in that comparatively large spaces are required for the light-projecting means and the light-receiving means. Further, the fabrication of the focusing, detecting optical system and the circuit system are complicated. As a result, the cost of such a system is high.
Accordingly, it is an object of the invention to provide a photosensor of relatively simple construction.
It is another object of the invention to provide a photosensor wherein a signal-to-noise ratio can be made large and focusing can be detected with a high precision.
It is another object of the invention to provide a photosensor which is able to detect focusing in a simple manner.
It is another object of the invention to provide a photosensor which can be made at relatively low cost.